Q.What do you mean by SI units ? What is the SI unit of mass ?

Accuracy is how close a measured value comes to the true (accepted) value of a quantity, and it is limited by the least count of the measuring instrument — the smallest quantity that instrument can reliably distinguish (e.g. 0.1 mL for a typical burette, giving a reading like $10.2 \pm 0.1$ mL). Error in a single measurement is expressed as absolute error (observed value − true value) or, more usefully, as relative error (absolute error ÷ true value, as a percentage). Precision, in contrast, is how closely a SET of repeated measurements of the same quantity agree with EACH OTHER, regardless of whether they are close to the true value; it is expressed through absolute deviation (a single reading minus the set's own mean), mean absolute deviation (the average of those deviations across the whole set), and relative deviation (mean absolute deviation ÷ the mean, as a percentage). High precision — tightly cluster …